1. Field of the Invention
The present invention relates to a ceramic heater used in an ignition or evaporation heater for various combustion apparatuses such as kerosene burning fan heater, heating element for various sensors, measuring instruments, electronic components, industrial equipment, home electric appliances, etc., water heater, heating element for air-fuel ratio sensor of automobile, carburetor heater for automobile, glow plug for internal combustion engine that quickly preheats an auxiliary combustion chamber when starting a diesel engine or during idling, or the like and a method for manufacturing the same.
2. Description of the Related Art
There have been various sheathed heaters comprising heat generating resistor consisting of a metal wire of high melting point embedded in a sheath made of a heat resistant metal and various ignition devices that utilize spark discharge, used as ignition or heating elements such as glow plug used for assisting quick starting of diesel engine. However, these heaters have such problems that it is difficult to quickly raising the temperature, and they are inferior in wear resistance and in durability. The ignition apparatuses, in particular, have such drawbacks as low reliability with regards to igniting without fail as well as the possibility of causing electromagnetic interference such as the noise generated at the time of ignition.
Accordingly, use of ceramic heaters have been increasing due to high heat transmission efficiency, capability to quickly raising the temperature, operation without electromagnetic interference, reliable ignition, high safety, high wear resistance and high durability. Ceramic heaters are used in wide applications such as the glow plug for internal combustion engine.
One type of ceramic heating element that is in common use is a ceramic heater comprising a heating element made of a metal having high melting point provided on the surface or inside of alumina ceramics member. This heater has a heat generating resistor made of a metal having high melting point such as W, Re or Mo buried in the ceramic member that is constituted from alumina as the main component as described in, for example, Patent Documents 1 through 4. The heat generating resistor is connected to lead members via electrode pad.
A ceramic heater having cylindrical shape is manufactured as described below. First, a ceramic core and a ceramic sheet are prepared. A paste of a metal having high melting point such as W, Re or Mo is printed on one side of the ceramic sheet thereby to form a heat generating resistor and a lead-out section. Then the ceramic sheet is wound around the ceramic core with the surface where the paste has been printed facing inside. When this assembly is fired to integrate, the ceramic heater is obtained.
The ceramic heater has a through hole formed therein, through which the electrode pad formed on the back surface of the ceramic sheet and the lead-out section are connected to each other. The through hole may be filled with an electrically conductive paste as required. The electrode pad 4 formed on the back surface of the ceramic sheet is connected with the lead members by means of a brazing material. The heat generating resistor is caused to generate heat by supplying electricity through the lead members.
While there is a type of ceramic heater 1 having external terminals pressed against the electrode pad without brazing lead members, the type of brazing the lead members is becoming dominant in the current trend of the market.
Since alumina (Al2O3) used as an electrically insulating material has low heat resistance and low strength at high temperatures, non-oxide ceramics, especially silicon nitride ceramics that is excellent in heat resistance, thermal shock resistance and oxidation resistance is also widely used. Silicon nitride ceramics is excellent in heat resistance, and has high strength at high temperatures, low heat capacity and good electrical insulation. Therefore, silicon nitride ceramics is used in wide applications to form the ceramic heating element that is capable of quick heating and is operated at high temperatures.
The silicon nitride ceramic heater has such a structure, for example, as described below. A substantially U-shaped heating element, leads and a lead-out section are embedded inside of a ceramic body formed from sintered silicon nitride in a round tube or cylindrical shape, and an end of the lead-out section is formed so as to be exposed on the surface of the ceramic body. As described in Patent Document 5, a metal plate made of Ni is bonded by means of a metal layer onto the lead-out section that is exposed at the end on the surface of the ceramic body, while the lead members are welded onto the metal plate made of Ni.
In case non-oxide ceramics such as silicon nitride is used, the leads are connected in a way different from the case of a ceramic heater formed from oxide ceramics such as alumina. When bonding a metal to ceramics, it is a common practice to metalize the ceramics surface by Mo—Mn method and, after plating the surface with Ni, the metal is bonded by brazing, active metal method, shrink fitting, pressure impregnation, glass welding process or other method. The Mo—Mn method and the active metal method, in particular, achieves relatively high strength of bonding without need for high precision treatment of the junction, and therefore have been employed in wide applications (See, Patent Documents 6 through 8). However, although the Mo—Mn method is widely applied to oxide ceramics represented by alumina, it is difficult to apply this method to non-oxide ceramics. As a result, non-oxide ceramics has been often bonded by the active metal method or the like.
Patent Documents
    1. Japanese Unexamined Patent Publication (Kokai) No. 11-354255    2. Japanese Unexamined Patent Publication (Kokai) No. 11-257659    3. Japanese Unexamined Patent Publication (Kokai) No. 2001-126852    4. Japanese Unexamined Patent Publication (Kokai) No. 2002-146465    5. Japanese Unexamined Patent Publication (Kokai) No. 7-25674    6. Japanese Unexamined Patent Publication (Kokai) No. 4-317473    7. Japanese Unexamined Patent Publication (Kokai) No. 6-1670    8. Japanese Unexamined Patent Publication (Kokai) No. 11-329676
Particularly for the ceramic heater used in automobiles, lead members having high bonding strength is in demand. However, conventional ceramics heaters have such a problem that tensile strength of the lead members decreases due to thermal cycle during use. When the lead member that has been brazed is left to stand in atmosphere at a high temperature, brazing strength decreases significantly in some cases. Also in case there is a significant difference in the thermal expansion coefficient between the ceramic body and the metal member that bonds the lead, residual stress is generated around the junction during a cooling process, and the residual stress sometimes acts onto the surface of the brazing material or ceramic body thereby decreasing the bonding strength